The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. On lower lead layer is seed layer 11 on which is antiferromagnetic layer 12 whose purpose is to act as a pinning agent for magnetically pinned layer 13. Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. Capping layer 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer remains fixed.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Earlier GMR devices were designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have begun to replace them. For CIP devices, the signal strength is diluted by parallel currents flowing through other layers whereas in a CPP device, the total transverse (series) resistance of all layers, other than the free layer, should be as low as possible.
It is known that AFM layer 12 together with high resistance seed layer 11 contribute most of the series resistance in a CPP-GMR structure. Although its functional unit (free/spacer/pinned layers) has a much higher GMR ratio, the entire CPP-GMR structure will have a low GMR ratio resulting from the large resistance of AFM/seed layer. Furthermore, the AFM/seed layers form hot spots that further limit the applied current density that can be employed.
In a related application (application Ser. No. 10/718,373 filed Nov. 20 2003), the CPP structure illustrated in FIG. 2 was disclosed. As can be seen, the free and cap layers have been patterned to have a lower width than the remainder of the stack so the resulting resistance of such a structure is greatly reduced. A typical spacer material such as Cu has an electron spin diffusion length about 1500 Å. Within this length, the spin directions of path-altered electrons remain unchanged so that ΔR can be maintained. The GMR ratio of the structure seen in FIG. 2 is increased due to the reduced series resistance and hot spots are also be eliminated, thereby allowing higher applied current density and increased signal.
To simplify the description, all the layers above the spacer layer are called the top CPP stack and the remaining layers are called the bottom CPP stack. Using conventional process techniques, this structure has been fabricated using two separate lithography/etching/lift-off sequences. The first step patterns the larger CPP bottom stack while the second step patterns the top CPP stack.
There are, however, several problems associated with this approach. To maximize the GMR ratio in the CPP structure shown in FIG. 2, both top and bottom CPP stacks must have sub-micron dimensions and they have to be precisely positioned. As the areal density increases, proper alignment at these continuously shrinking dimensions becomes very difficult. Furthermore, after the first lithography/etching/lift-off sequence, the etched CPP structure is exposed to the environment and is subjected to attack from moisture or chemicals.
The present invention discloses a novel method which will eliminate the above problems. This technique will also improve edge profiles while achieving small-dimension alignment so that the desired spin diffusion length can be obtained on both sides of a CPP GMR sensor.
A routine search of the prior art was performed with the following references of interest being found:
Pang et al., in U.S. Pat. No. 6,496,334, describe using IBE in etching the CPP stack. In U.S. Pat. No. 6,294,101, Silverbrook discloses IBE rotation during etching. Lederman et al (in U.S. Pat. No. 5,627,704) and Dykes et al. (in U.S. Pat. No. 5,668,688) are of interest as having to do with CPP fabrication, but do not mention the IBE etching of the present invention.